Prospect of growing human organs for transplantation raised by creation of first ever embryos combining two large, distantly related species

Chimeric Organisms Hold Promise for Science and Medicine

Scientists have created a human-pig hybrid in a milestone study that raises the prospect of being able to grow human organs inside animals for use in transplants.

It marks the first time that embryos combining two large, distantly-related species have been produced. The creation of this so-called chimera – named after the cross-species beast of Greek mythology – has been hailed as a significant first step towards generating human hearts, livers and kidneys from scratch.

Juan Carlos Izpisua Belmonte, who led the work on the part-pig, part-human embryos at the Salk Institute for Biological Studies in La Jolla, California, said: “The ultimate goal is to grow functional and transplantable tissue or organs, but we are far away from that. This is an important first step.”

The study has reignited ethical concerns that have threatened to overshadow the field’s clinical promise. The work inevitably raises the spectre of intelligent animals with humanised brains and also the potential for bizarre hybrid creatures to be accidentally released into the wild. The US National Institutes of Health (NIH) placed a moratorium on funding for the controversial experiments last year while these risks were considered.

Izpisua Belmonte said that fears around chimeras were inspired largely by mythology rather than the realities of meticulously controlled experiments. But he acknowledged: “The idea of having an animal being born composing of human cells creates some feelings that need to be addressed.”

The paper, published in the journal Cell, outlines how human stem cells were injected into early-stage pig embryos, resulting in more than 2,000 hybrids that were transferred to surrogate sows. More than 150 of the embryos developed into chimeras that were mostly pig, but with a tiny human contribution of around one in 10,000 cells.

The pig-human embryos were allowed to develop to 28 days (the first trimester of a pig pregnancy) before being removed.